•Player Lock: To provide a fair matchmaking experience, any progression on units (ex. Promotion) that occurs after the preview phase has ended will NOT be reflected on that unit's progression in this event. For example, if the unit was 4-star at the moment of Player Lock, it will remain 4-star within this event even though it is 5-star in other areas of the game. This is to prevent a variety of exploits such as removing all mods before matchmaking, getting a weaker match, and then replacing them all.

MistPassiert in the teamwork video says that the lock-in happens when you join the war. That way you can set your mods as you want them for the war, join it, then move them for other things. (i.e. do it at your convenience, not having to leave them a special way at an arbitrary time). If you do major updates, you need to leave the war and rejoin it to get them reflected.
However, I do not know if that is correct. The way I would have read some of the other material is that the lock-in happens at the end of the preview phase. I would also be interested in knowing what really happens.

I did one test: during the preview phase, after joining I raised my gp. I then left the TW, and rejoined it. The active GP went up from what it had been before I left. I assume this reflected my changed GP. (Obviously I don't know what else might have been happening, somebody else might have updated something elsewhere which changed the GP, but that seems unlikely.)

It locks out changes after the preview phase, so you can join it but once that 24 hour preview phase is done and goes into the defensive phase that's when the changes are no longer reflected in the TW and the GP.